In this work, a new and highly active heterogeneous Fenton system based on iron metal and magnetite Fe-0/Fe3O4 composites has been prepared by controlled reduction of iron oxides. Temperature-programmed reduction experiments with H-2 showed that iron oxides, i.e. Fe2O3, FeOOH and Fe3O4, can be reduced to produce highly reactive Fe-0/Fe3O4 Composites with different metal to oxide ratios as determined by Mossbauer spectroscopy and powder X-ray diffraction. Mossbauer measurements revealed that these composites are reactive towards gas phase molecules and can be oxidized rapidly by O-2 even at room temperature. The composites showed also very high activity for the Fenton chemistry, i.e. the oxidation of an organic model contaminant, the dye methylene blue, and the H2O2 decomposition. The best results were obtained with the composites with 47% Fe-0 obtained by reduction of Fe3O4 with H-2 at 400 degrees C for 2 h, which produced a very rapid discoloration with total organic carbon (TOC) removal of 75% after 2 h reaction. Conversion electron Mossbauer spectroscopy (CEMS) measurements before and after H2O2 reaction showed that Fe3O4 and especially Fe-0 are oxidized during the reaction. The reaction mechanism is discussed in terms of the formation of HO center dot radicals by a Haber-Weiss initiated by an efficient electron transfer from the composite Fe-0/Fe3O4 to H2O2. The higher activity of the composites compared to the pure Fe-0 and iron oxides has been explained by two possible effects, i.e. (i) a thermodynamically favorable electron transfer from Fe-0 to Fe3O4 producing Fe-magnetite(2+) active for the reaction and (ii) by the formation of very reactive small particle size Fe-0 and Fe3O4. (C) 2008 Elsevier B.V. All rights reserved.